If you are a freelancer , a consultant, or a small business owner who does a lot of sales calls, the estimate and the invoice are simply facts of life. An app that lets you use your iPhone or iPod touch to generate professional invoices, estimates, purchase orders, and credit memos for your clients in a snap would be a valuable utility. Invoice2Go from Invoice2go.com proposes to be such an app, yet falls short in the most obvious way to just about anyone reading this on Macworld.com.